To get the model into the game, you need
Dom's exporter. It is as straightforward as it gets. But a quick look into the manual is always needed.
The game will crash if there are untextured faces, so there is a certain minimum of texturing needed. Also the material must be set up. Regarding the material, see Dom's manual.
Regarding unwrapping, this is the minimum you need to do:
1) In edit mode, select all faces and press u for unwrap, then reset.
2) Create a squared image of any solid colour. E.g., take a screenshot of anything blue or black or whatever on your screen and crop that to 16x16 pixels. Save as TGA and convert to ACE, using RWAcetool.
3) Put this image (both TGA and ACE for good measure) into a subfolder named textures in the folder where your .blend file resides. Also copy that subfolder to the Source folder in the railworks file hierarchy.
4) Make sure your material refers to this file, using a relative path.
Using blueprint editor, create a scenery blueprint using the .igs that was produced by the exporter. It should show in game, all blue or black or whatever.
Once you got that, you will want different colours. The not-so-performance-friendly way would be to define different materials, one for each colour.
The much better way is to create a grid of colour patches in a single image. I once used Excel to make squared cells of different background colour. Worked great and matched my artistic skills.
In Blender, load the image in a panel that uses UV view (image view).
Now, select all the stuff you want in yellow. Press u and select reset if you have not done that before. Then, while the mouse is over the image panel, press s for shrink and move the mouse to shrink the bag of lines you see. Then press g for grab and move them to the yellow area of the image.
One thing to keep in mind with the unwrapping business is that you can save some time if you do not postpone the unwrapping pain to the end but make a few unwrapped pieces first and then build your model from copies and modifications of these pieces. Just be warned that new faces will be untextured (i.e., not yet unwrapped) and create errors in the game if you don't spot them. But reshaping is not an issue.